Polyblends are mixtures of structurally different polymeric materials. They are generally prepared by melt-blending the polymers on an open roll, in an extruder, or in a suitable mixer, at a processing temperature above the highest glass transition temperature, in the case of amorphous polymers, or the highest melting temperature, in the case of semicrystalline polymers, of any polymer in the mixture. They are also routinely prepared by coprecipitation from solution.
Most polymers are immiscible with other polymers. A mixture of such materials possesses multiple phases, each of which typically exhibits its own thermal transition. Nevertheless, immiscible polyblends have great commercial importance, because many such blends have very desirable properties and very good cost:performance ratios.
In the case of immiscible systems, an approach to mixing is to form so-called interpenetrating networks. This is accomplished by swelling a crosslinked polymer with a monomer which is subsequently polymerized in situ. A similar approach has been taken to prepare mixtures of polyimides with a number of thermoplastic polymers such as polycarbonates, polyamide-amides, and polysulfones. A typical procedure is to polymerize a dianhydride and a diamine to the polyamic acid in the presence of a thermoplastic polymer solution. The mixture is subsequently given further thermal treatment to produce the insoluble infusible interpenetrating network of the polyimide and the starting thermoplastic.
A few polymers are miscible with other polymers. A mixture of such polymers possesses a single amorphous phase which exhibits a single glass transition temperature intermediate between those of the constituent polymers. A miscible polyblend offers the advantage of processing ease relative to the high glass transition constituent alone.
Examples of known miscible polyblends are mixtures of poly(vinylchloride) with copolymers or terpolymers of ethylene or with polyester oligomers, and mixtures of polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide), sold under the tradename NORYL. Other examples are blends of certain aromatic polybenzimidazoles and aromatic polyimides, discussed in the article by Leung et al., Polymer Bulletin, 16, 457-464 (1986).
New polyblends, whether immiscible or miscible, which offer advantages in terms of cost, properties, processing ease, or any combination of these, are constantly being sought by researchers in polymer science.